1. Field of the Invention
The present invention relates to an image forming apparatus of the electrophotographic type for forming an electrostatic latent image on a photosensitive body and for developing the resultant electrostatic latent image with toner.
2. Related Background Art
As means for obtaining a hard copy (such as a copying machine, a computer, or the like) image forming apparatus utilizing an electrophotographic method have been widely used. A typical image forming apparatus has a photosensitive body and various devices for image formation arranged around the photosensitive body. More specifically, around the photosensitive body, there are provided a charger device, an exposure device, a developer device, a transfer device, a cleaning device, and so on. This image forming apparatus of an electrophotographic method carries out an image forming process comprising steps of charging the photosensitive body with electricity, exposing the photosensitive body to light in order to form an electrostatic latent image on the photosensitive body, developing the resultant electrostatic latent image by applying toner to it to obtain a toner image, transferring the developed toner image onto a transfer medium, and subsequently fixing the transferred toner image on the transfer medium to finally obtain a print image.
Compared with print images obtained from other means for obtaining hard copies such as those of the thermal transfer type, the ink jet type, the impact printing type, or the like, the print image obtained as described above has higher resolution and stronger contrast, that is, as a whole, higher quality.
However, as described before, the image forming process by the electrophotographic method requires many devices. So, the apparatus therefor tends to be of a large size and complicated. It is not easy to miniaturize and simplify the apparatus.
In order to solve this problem, some methods have been proposed in which, while using the same electrophotographic method, the apparatus carries out all the processes such as electrification, exposure development, and so on substantially at the same time and at the same position (such combined processes will be referred to herein as a "simplified process"). Among such methods, typical ones are disclosed, for example, in Japanese Laid-Open Patent Appln. Nos. 58-153957, 62-209470 and the like. In general, in these methods, either conductive toner or conductive carriers, and insulating toner are used, and the image forming process comprises steps of (1) cleaning the residual toner which was not transferred in a previous image forming process; (2) contact electrification; (3) image exposure from the back surface of the photosensitive body; and (4) contact development. The series of steps are performed in a developing nip between the photosensitive body and a magnetic brush roller which corresponds to an exposure position on the back surface of the photosensitive body and which is in contact with the outer surface of the photosensitive body.
More specifically, as shown in FIG. 2, a magnetic brush provided upstream in the developing nip N between a developer sleeve 22 and a photosensitive body 1 scrapes off the residual toner which was not transferred (hereinafter referred to as "transfer residual toner") to clean the photosensitive body 1. As the toner employed here is a magnetic toner T and a fixed magnet 23 is arranged inside the developer sleeve 22, magnetic force can improve the cleaning effect.
Then, the surface of the photosensitive body 1 is brushed by a conductive magnetic brush (of conductive toner or conductive carriers) to apply an electric charge to the surface of the photosensitive body 1. As the electrification is carried out by trapping electric charge in impurity levels on the surface of the photosensitive body 1, charger member(s) having very small resistance and a long period of electrification are required to carry out electrification sufficiently. Therefore, a material which sufficiently holds electricity near its surface is needed. Amorphous silicon (hereinafter referred to as "a-Si"), selenium, and so on are preferably used.
The above-mentioned cleaning operation and electrification are performed at the same time in a cleaning-electrification region Nc, which is in the developing nip N and upstream with respect to a back surface exposure position A (described later). Incidentally, the potential of the charged photosensitive body 1 brushed with the magnetic brush is substantially equal to the applied voltage or less.
Next, the back surface of the photosensitive body 1 is exposed to light. A light source (exposure means) 3 having an LED array 31 illuminates a predetermined position (back surface exposure position) in the developing nip N formed by developer between the developer sleeve 22 and the photosensitive body 1. Thus, a latent image is formed on the exposed photosensitive body 1. The latent image is developed in a development region N.sub.D, which is downstream with respect to the back surface exposure position A, in the developing nip N. When conductive toner is used, the electric charge electrostatically induced by the latent image formed on the photosensitive body 1 is applied via a triboelectric brush to the toner at the tip of the triboelectric brush. The latent image is developed with toner separated from the triboelectric brush by a Coulomb force acting between said electric charge and the electric charge of the latent image.
Otherwise, when a two-component developer consisting of magnetic conductive carriers C and insulating toner T is used in the same apparatus, the triboelectric brush of the conductive carriers serves, as neighboring electrodes. Accordingly, sufficient electrical field for development can be obtained even if the voltage applied between the photosensitive body 1 and the developer sleeve 22 is small. Thus, development with insulating toner can be carried out by applying low voltage.
Since it is difficult to transfer a toner image in the electric field obtained when conductive toner is employed, development with a two-component developer including insulating toner is generally preferred.
However, in the image forming process in of prior art described above, some problems arise because a plurality of processes are carried out in the developing nip N between the photosensitive body and the developer sleeve.
First, in the contact electrification process with the conductive magnetic brush, electric charge is supplied to the surface of the photosensitive body, as described above. So, the material used for the photosensitive body 1 should have a level on the surface in which electric charge can be trapped. In this case, it is difficult to use a conventional organic photo-semiconductor (OPC) although such semiconductor has no toxicity and can have functions separately.
FIG. 3A shows a case in which the OPC photosensitive body 1 was electrified with conductive particles having a volume resistivity of 10.sup.4 .OMEGA. . cm. In this case, when voltage of -60 V was used for electrification, the photosensitive body 1 could be charged only with voltage of -40 V. Before the electric charge converged to -40 V, the photosensitive body 1 in the apparatus shown in FIG. 2 had to make five rotations. The cleaning operation is also influenced. In this process, as described above, a cleaning operation is performed in the cleaning-electrification region Nc upstream in the developing nip N between the photosensitive body 1 and the developer sleeve 22. Actually, the cleaning operation and electrification are performed at the same time.
The Van der Waals force between the toner and the photosensitive body 1 and the Coulomb force both act on the residual toner to be removed which was not transferred. But, Coulomb force more strongly acts on the toner developed in the exposed portions than the toner in the non-exposed portions. Accordingly, when the surface of the photosensitive body 1 is electrified with the conductive magnetic brush, a cleaning operation can be performed more effectively. However, if the photosensitive body 1 does not have a good electrification characteristic, it can not be sufficiently charged in the cleaning-electrification region Nc, which causes cleaning failure and a positive ghost image. The positive ghost image may be generated not only by cleaning operation failure in which the residual toner used for the previous image forming process is not sufficiently removed, but also may be generated by an electrification failure caused by such residual toner.
On the other hand, when the a-Si photosensitive drum 1 coated with a silicon-calcium carbide layer was electrified in the same way, it could be charged with 55 V, as shown in FIG. 3B after making only two rotations of electrification. At the same time, in this reverse development system, the surface of the photosensitive drum 1 and toner are homopolar. So, repulsion forces between them remarkably improved the cleaning effect.
In short, the above-mentioned first problem is that the material used for the photosensitive body 1 can not be selected freely but must be selected from a comparatively few materials that realize a good electrification characteristic.
Secondly, even if the a-Si photosensitive body 1 is employed, a conductive magnetic brush whose resistance value is 10.sup.3 .OMEGA. or more can not supply sufficient electric charge, which causes electrification failure. So, it is preferable to use materials for the photosensitive body 1 which have even better electrification characteristics.
Thirdly, since the a-Si photosensitive body 1 is prepared by deposition and other processes, the transparent substrate used for the photosensitive body 1 should be made of an expensive heat resistance material. Accordingly, it is desirable that the photosensitive body 1 should be made of a less expensive material, such as OPC.